This document specifies a reversible mapping (or transformation) from Rule Interchange Format (RIF) XML documents to Resource Description Framework (RDF) graphs. This mapping allows the contents of RIF documents to be interoperably stored and processed as RDF triples, using existing serializations and tools for RDF. When used with the standard mapping from RDF triples to RIF frames, this also provides a "reflection" or "introspection" mechanism, an interoperable way for RIF rules to operate on RIF documents.

May Be Superseded

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

No Endorsement

Publication as a Working Group Note does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

RIF was envisioned [RIF Charter] to be extensible, allowing third parties to define non-standard extensions which could be combined into new dialects, as needed, to support interchange of rule sets which include features not defined in the standard dialects. Despite this vision, no general mechanism for extensions has been detailed in the RIF specifications. The RIF Framework for Logic Dialects (FLD) [RIF FLD] does, however, specify a way to create more expressive logic dialects.

This specification defines a reversible mapping from RIF syntactic structures to RDF graphs. The definition is presented in tables where each row in the tables shows an XML template and a corresponding RDF graph template. The mapping is performed, roughly speaking, by finding the first matching XML template, then producing the corresponding graph. In some cases, the graph will require recursive translations of XML subtrees. The resulting graph has one node, called the focus node, which represents the XML root node, which in RIF Core, BLD, and PRD is rif:Document.

A reverse mapping, described in section 6, is possible for standard RIF by simply matching the RDF template and generating the corresponding XML. For extended RIF, so the reverse mapping can only done if the translator knows the XML grammar being generated.

Note that RDF serializations produced via this mapping are not standard RIF documents and cannot necessarily be understood by RIF implementations.

The rest of this document is structured as follows:

design discussion, including use cases and requirements

specification of the RIF-in-RDF mapping

discussion of the reverse mapping

specification of rif:usedWithProfile, allowing import of RIF into RDF

a complete example

2 Use Cases

In designing this mapping a few use cases were considered:

UC1: Store RIF in an RDF Triplestore — particularly when using RIF with RDF data, it may be useful to keep various RIF documents in the triplestore with the data, especially when there is associated metadata.

UC2: Access RIF Syntactic Structures with RDF Tools — even more than storing whole RIF documents, it may be useful to be able to use RDF and Linked Data mechanisms to refer to and manipulate individual syntactic elements such as RIF rules, clauses, and groups.

UC3: Transform RIF Syntax using RIF Rules — many logic programming techniques build on the idea of having rules which transform other rules. While RIF documents can be processed as XML, it may be desirable in some cases to process them as RDF triples or as RIF frames.

UC4: Provide Forward Compatibility via Fallback Rules — it may be possible for RIF extensions to be completely or partially understood (used) by systems which do not directly implement the extensions, if the extensions are published with suitable fallback transformation rules. This is a special case of UC3.

Req2: Extensions Can Be Written So They Will Be Automatically Mapped to RDF — It is possible to write reasonable extensions which the mapping will handle, without the mapping being extended. It is not a requirement that all possible extensions be handled by this mapping.

Req3: Transformations Require No External Data — The transformation can be done without any external information, such as dereferencing namespace URIs or otherwise obtaining schema information.

Req4: Stable Roundtrips Under RDF Simple Entailment — A RIF document may be mapped to RDF, then the graph may be altered following RDF Simple Entailment [RDF Semantics] (including being reduced to a subgraph), and if the document can be extracted by the reverse mapping, it will have the same entailments and metadata. This is motivated by UC3 and especially UC4: without this property, incomplete running of transformation rules could undetectably result in incorrect results.

Req5: RDF View Conforms to RDF Best Practices — the RDF form of the RIF constructs should appear as normal, well-constructed RDF data, not as some odd or surprising formation. Although this mapping uses rdf:List structures more than is common, for this application they are warranted.

Req6: RIF Extension are First Class in RDF View — viewed as triples, there should be no indication of which features are in which dialects or extensions; the intent here is the allow the feature set to evolve and particular applications to use the appropriate set of features without regard to which features happen to be in RIF Core, RIF BLD, or RIF PRD.

4 Extensibility

Conceptually, in RIF there are two kinds of extensions, divided by how consumers which do not implement the extension are to handle them. Extensions which non-implementing consumers may ignore are encoded in RIF metadata; extensions which non-implementing consumers must understand before processing require altering the syntax so that the RIF document will not be schema valid, such as by introducing new XML elements.

In the RDF mapping, this distinction must be made somewhat differently. In RDF, may-ignore extensions can be encoded with additional triples about RIF syntactic elements, while must-understand extensions require removing or replacing properties required for non-extended decoding. This absence of a required arc prevents the decoding to a schema-valid XML RIF document, causing non-implementing consumers to abort.

These restrictions are necessary in order to meet the stated requirements. In particular, without these restrictions, a RIF document (in RDF graph form) being transformed by an incomplete reasoner into another RIF document (also in RDF graph form) could produce an unintended and incorrect result just because the reasoning was incomplete. With these restrictions, the result will not match the reverse-mapping until it is sufficiently complete.

5 Mapping from RIF XML to RDF Graphs

The mapping from RIF XML to RDF Graphs is expressed as a function Tr:

Tr(rif-xml-tree) → <focus-node, triples>

For every standard RIF Document, and for certain subtrees of RIF documents and extended RIF documents, Tr maps to the pair of an RDF node and an RDF graph. The node, called the focus node represents the same syntactic element as the root of the given XML tree. The RDF graph is a standard RDF graph, a set of RDF triples, and always contains the focus node. The focus node is usually a fresh blank node, but it might have a IRI label in certain cases, as detailed below.

In this document, the Turtle [Turtle] RDF serialization syntax is used for expressing triples and graphs. Turtle has a very terse syntax for lists, ( item-1 ... item-n ) and for fresh blank nodes and the triples using them as the subject: [ property-1 value-1; ... property-n value-n ]. These constructs allow the mapping to be presented and examples to be shown with relative simplicity.

The mapping is defined recursively, with each application of Tr converting an XML class element to a focus node, with additional triples. Class elements in RIF XML have tags that begin with an uppercase letter and represent a particular syntactic entity. Except for rif:Var and rif:Const class elements (detailed in Table 1, below), all the class elements follow a general form, containing a sequence of property elements, each containing additional class elements. The mapping for these is detailed in Table 2 and Table 3, below.

If a system performing the mapping has determined the input document to be in one of the three standard dialects (Core, PRD, BLD), the rdf:type of the root focus-node may be set corresponding to that dialect: rif:CoreDocument, rif:PRDDocument, or rif:BLDDocument, instead of rif:Document. These classes for the standard dialects are defined to be disjoint: rif:BLDDocument contains those BLD documents which are not Core documents, and rif:PRDDocument contains those PRD documents which are not Core documents. Extensions may result in additional rdf:type arcs on the root focus-node.

Note that RIF XML allows for relative IRIs, which are expanded to absolute IRIs using the xml:base directive. This expansion must be done before Tr.

5.1 Namespaces

All standard elements in RIF XML have the namespace "http://www.w3.org/2007/rif#", and the attributes have no namespace. Extensions are expected to use other namespaces for the elements and are not allowed to introduce new attributes.

The RIF-in-RDF mapping produces RDF graphs that use the same namespace, although they use that namespace name in the normal RDF way (as an IRI prefix) instead of in the XML way (as a disambiguator).

By keeping the namespace the same, transformation software can correctly operate, without modification, on all RIF documents, even ones containing extensions.

Note that this use of the same namespace means that in certain cases RIF and RDF/XML documents cannot be distinguished simply by their namespace use. Moreover, since the rdf:RDF root element is optional in RDF/XML, in some cases it is not possible to distinguish between RIF and RDF/XML documents just by schema-validating or RDF-parsing the XML. In those cases, additional inspection of the structure is necessary. In general, systems should therefore be careful to maintain external file type information. This is typically done with either the media types ("application/rif+xml" and "application/rdf+xml") or the suggested filename extensions (".rif" and ".rdf").

In the tables below, the followng XML DOCTYPE declaration is assumed, allowing for abbreviation of the RDF and RIF namespaces:

5.2 The <id> and <meta> Elements

When this child is present, it is ignored for other processing and the id text is used as the IRI (URI-Reference) label for the focus_node, instead of it being left as a blank node.

As a special case, if the rif:Document element does not have an <id> child, its focus_nodeshould be given the Web address (IRI) of the input XML document, if it has one.

After this optional <id> element, any RIF class element may contain a <meta> element. This element is processed according to general element processing rules, below.

Systems performing this transformation may also attempt to convert the metadata to RDF using the standard Frame-RDF correspondence [RIF RDF+OWL] , and include it in the returned triples. The conversion is not always possible, because some frame formulas are not expressible in RDF; systems which attempt this transformation and encounter such frame formulas in metadata should issue a warning. Even if the frames are converted to RDF like this, implementations must, under default settings, still keep the rif:meta triples intact, to support stable roundtripping.

5.3 The <Var> and <Const> Elements

Table 1, below, defines a portion of Tr. When a rif-xml-tree, X, matches the entry in column one, treating terms written like-this as metavariables, the result of Tr(X) is the pair <focus_node,G>, where G is the set of triples indicated by the second column.

Note that, although not shown in this table, <id> and <meta> child elements are allowed before the text in these elements. Additional elements after <id> and <meta> and before the character data may be allowed by extended schemas and must be processed as normal (extended) property elements.

Note that metadata about Consts is applied to the focus_node, not to the RDF literal produced. For example an explanation comment on a Const is understood to explain why that value is used in that spot, not state general properties of that value.

Table 1: RIF-RDF Tr Mapping Table for Var and Const

Input XML Pattern, X

Output RDF Triples, G

<Var>variable-name</Var>

focus_node rdf:type rif:Var
focus_node rif:varname "variable-name"

<Const type="&rif;iri">value</Const>

focus_node rdf:type rif:Const
focus_node rif:constIRI "value"

Note that the value is an absolute IRI. Relative IRIs must be converted using the xml:base.

<Const type="&rif;local">value</Const>

focus_node rdf:type rif:Const
focus_node rif:constname "value"

<Const type="&rdf;PlainLiteral">text@</Const>

focus_node rdf:type rif:Const
focus_node rif:value "value"

<Const type="&rdf;PlainLiteral">text@langtag</Const>

focus_node rdf:type rif:Const
focus_node rif:value "value"@langtag

<Const type="type-iri">value</Const>

focus_node rdf:type rif:Const
focus_node rif:value "value"^^type-iri

5.4 General Mapping

Except as noted above, the Tr mapping for any class element (denoted as parent in Table 2) is to a new focus node and a set of triples which depend on each of the children of the class element. The exact dependency is detailed in this section.

Each child of the class element being mapped (except as noted above) is a property element (denoted as child in Table 2). There are four kinds of property elements:

Mode 0

These elements have the ordered="yes" attribute. Their children are mapped to an RDF list (collection).

Mode 1

These elements are required by the XML schema to appear exactly once. Their children are mapped directly to the value role of an RDF triple

Mode 2

All the (zero or more) values of these elements are gathered, in document order, into an RDF list. When these elements do not appear in their class elements, an empty RDF list is generated.

Mode 3

Special handling for the <slot> property, converting name/arg and key/value pairs into explicit pairs

The mapping for each mode is specified in Table 2 below. The mapping depends on the identity of an RDF property, written as prop, and the mode. Table 3 specifies special-case values for prop and mode, but otherwise they are determined as follows:

prop is the concatenation of the property element's tag's namespace IRI followed by its local part. For example, for the <rif:args> element, the RDF property prop has the IRI "http://www.w3.org/2007/rif#args".

If the element has an attribute "ordered" with the value "yes", it is Mode 0; otherwise, it is Mode 1. (As noted, RIF extensions must use required property elements, so Modes 2 and 3 are not available to them.)

As a special case, if item is merely character data (with no XML elements), id is an RDF plain literal with the same value and triples-n is empty. This occurs in RIF Core, for example, with the rif:location property element.

This table specifies exceptions to the default rules for determining the value of prop and the mode of the property element:

Table 3: Mapping from RIF Parent/Child XML Elements to RDF Properties

Class Element (parent)

Property Element (child)

RDF Property (prop)

Mode

Document

directive

rif:directives

2

Group

sentence

rif:sentences

2

Forall

declare

rif:vars

2

Exists

declare

rif:vars

2

And

formula

rif:formulas

2

Or

formula

rif:formulas

2

Frame

slot

rif:slots

3

Atom

slot

rif:namedargs

3

Expr

slot

rif:namedargs

3

6 The Reverse Mapping (Extracting RIF XML)

We call the inverse mapping XTr:

XTr( focus-node, triples ) →rif-xml-tree

For any RIF-XML document D, valid according to some RIF dialect XML schema, given the transformation:

D' = Xtr( Tr(D) )

D' and D will differ only in presentation aspects (non-significant whitespace, XML comments, IRIs being made absolute using xml:base, etc),
unrelated to the semantics. That is, the semantics of D will be
preserved so the rule sets corresponding to D and D' will have
identical entailments under the relevant RIF Dialect semantics.

The ordering of child property elements by Xtr is determined by the relevant dialect schema, if known; otherwise they are lexicographic, sorted first by namespace then by local part. Xtr implementations may accept schema parameters to
constrain their extraction to conform to a particular
dialect's schema, and handle non-lexicographic ordering, like:

If they do not do this, they will not be able to correctly extract RIF
XML documents in non-standard schemas using non-lexicographic element
ordering. (For this reason, extensions are advised to use lexicographic ordering of elements in their schema.) For the three standard dialects, Xtr implementations
must emit the children in schema-valid order.

7 Importing RIF into RDF

We hereby define an RDF predicate rif:usedWithProfile which enables an import to be specified from the graph G instead of from R. This definition also appears in SPARQL 1.1 Entailment Regimes [SPARQL ER].

In the simple usage the graph G is a plain RDF graph and rif:usedWithProfile is used to combine that graph with one or more externally defined RIF rule sets. In this usage each subject of a rif:usedWithProfile assertion should be the URI for a RIF rule set (which may be encoded in RIF-XML or RIF-in-RDF) and the object should be an import profile as defined in RIF RDF and OWL Compatibility [RIF RDF+OWL].

It is also possible for the graph G to itself contain an encoded ruleset along with additional RDF statements to which the ruleset is intended to apply. If graph G is obtained from a base IRI Ug then the statement:

Ug rif:usedWithProfile P .

within the graph causes it to be treated as both the graph G and the source of the rule set R in the combination (R, G). Syntactically such a statement can be made by using the empty relative IRI reference <> provided that the document base IRI has been set appropriately.

The semantics of rif:usedWithProfile is explained in the following section.

Where Ri and Pi are the subjects/objects respectively of triples of
form:

Ri rif:usedWithProfile Pi

and Ri' denotes

the RIF document Ri if Ri is a RIF/XML document, and

the RIF document obtained from applying the inverse mapping XTr to the graph Gi if Ri denotes an RDF graph Gi.

Remark 1:
Note that the fact that G' is treated as being imported with all profiles P1 ... Pn enforces G' to be treated according to the highest profiles among P1 ... Pn, cf. Section 5.2 of [RIF RDF+OWL].

Remark 2:
If G also includes a rif:usedWithProfile statement referring to itself (i.e. with subject
Ug where the graph G can be obtained from Ug) then the rules encoded in that document will be included in the rule imports but the encoding of the rules will remain visible within G'.

Remark 3: Note the discussion in the section 6 that the inversion of Tr
is not a deterministic function.

Remark 4:
Note that in the case where the graph G includes encoded RIF
rules then, as a result of RDF graph merge, it may encode more than one
RIF document. A RIF-in-RDF process MAY choose to combine all the rules
in each documents into a single RIF document or MAY issue a warning.
Note that future RIF dialects may have semantics which depend on rule
ordering.

9 Acknowledgements

This document is the product of the Rules Interchange Format (RIF) Working Group (see below) whose members deserve recognition for their time and commitment. The editor extends special thanks to Dave Reynolds for his particularly attentive and insightful review comments.